U.S. Pat. No. 2,232,099 (de Jahn) indicates that carbon is needed to reduce phosphogysum, but he recognized that silica would work only if alumina were present.
U.S. Pat. No. 2,687,946 and 4,687,947 (Manning) similarly teach that carbon is an essential component for the subject process.
Sulfur dioxide (SO2) is released and recovered from calcium sulfate (CaSO4) while generating a usable, environmentally friendly by-product, calcium silicate (CaSiOx). A stochiometric amount of CaSO4 particles is admixed with silica (SiO2), e.g. in the form of sand, waste by-products, etc., and then introduced into a fuel-fired kiln.
Feeding water vapor into the kiln is critical to making the process work correctly. The water vapor is partially dissociated into nascent hydrogen (H0) and nascent oxygen (O0) at an elevated temperature in the kiln. The water vapor for the process is supplied by either direct water sprays or by steam injection.
At the elevated temperature of the kiln, the SiO2, H0 and O0 react with the CaSO4 to form CaSiOx and released SO2 vapor. The SO2 vapor exits the kiln in a discharge gas stream and is captured and recovered as sulfuric acid (H2SO4) in a typical commercially available tail-gas absorption/recovery unit, similar to those used in smelter operations.
Once the CaSiOx is formed, it passes through the kiln and is discharged and transported to outside storage stacks. As the CaSiOx is tumbled inside the hot kiln, it tends to granulate into mostly smooth particles that have a glass-like coating on the outside. All impurities that were contained in the CaSO4 are trapped inside the glass-like coating. Depending upon the impurities in the CaSO4, the glass-like coating can effectively prevent/reduce release of objectionable gaseous elements, such as radon. The CaSiOx particles are essentially insoluble and can be safely and effectively utilized for construction fill material, such as roadways, or as aggregate in cement. If the material is not sold, it can be stored in outside stacks that are environmentally friendly.